rg2200
+1y
If you want to maintain the same outside circumference (to keep the speedo etc. correct), then go here:
and enter the stock tire size, then pick 18" as the new tire and adjust until you get it right or close.
For example, stock 205/74R14 matches within .1" circumference with a 225/45R18.
If you want a wider tire, then a 245/40R18 is pretty close. Stock, narrrow, width of 205, you'd need a 205/50R18.